1. Field of the Invention
The present invention relates generally to microstrip antennas and more particularly to a microstrip antenna having a circular radiation element.
2. Description of the Prior Art
It has been proposed that a wireless communication system is established between a base station and a number of mobile stations via a geostationary satellite (see Japanese Pat. Application No. 63-331494).
FIG. 1 shows such a previously-proposed wireless communication system, in which a down channel between a base station CS and a number of mobile stations M is established via a geostationary satellite STd, while an up channel between the mobile stations M and the base station CS is established via a geostationary satellite STu. The frequencies of the up channel and the down channel are selected to be, for example, 1.6 GHz and 4.2 GHz, respectively. In this wireless communication system, a user HQ such as a transportation company and the base station CS are connected via another communication network line L, by way of example.
In the above-noted wireless communication system, the mobile station M side utilizes a microstrip antenna because it is simple in construction and has a low physical profile.
The microstrip antenna according to the prior art will be described with reference to FIGS. 2 and 3.
As shown in FIGS. 2 and 3, a circular radiation element 3 is laminated (i.e. stacked) on a rectangular ground plane conductor element 1 via a dielectric element 2 made of a material such as a fluoroplastics having a low dielectric loss. A feed point 3f is located at a position offset from the center of the circular radiation element 3, and is connected with an inside conductor 5 of a coaxial feed line 4. Reference numeral 6 designates an outside conductor forming the coaxial feed line 4.
When the circular radiation element 3 in this microstrip antenna resonates in the TM.sub.11 mode (i.e. waveguide dominant mode), a surface current is distributed as shown by dashed lines in FIG. 2, and a directivity of the radiation becomes unilateral in which a maximum gain is provided in the front direction.
In the mobile wireless communication system utilizing a geostationary satellite or the like, the elevation angles of the geostationary satellite as seen from a mobile station falls within a range of from about 25 to 65 degrees in mid-latitudes.
When the prior-art microstrip antenna as described above is used in the mobile station side, the maximum gain direction of the antenna and the elevation angle of the geostationary satellite do not coincide with each other, degenerating the antenna gain.
In order to obtain a desired directivity that is matched with the angle of elevation of the geostationary satellite, it is generally proposed to provide a microstrip array antenna in which a plurality of microstrip antennas are properly connected to feed radiation elements with different phases.
This type of microstrip array antenna is, however, increased in size and becomes complicated in structure.
The mobile station side in the above-noted wireless communication system needs independent antennas respectively corresponding to the up channel and down channel.
IEEE Transactions on Antennas and Propagation (Vol. 27, No. 3, pp. 270 to 273, published on March, 1978), for example, reports a two-frequency antenna in which a non-feed circular conductor element is coaxially stacked (i.e. laminated) on the radiation element 3 of the prior-art microstrip antenna (shown in FIGS. 2 and 3) via the dielectric element.
This two-frequency antenna cannot cover two frequencies (1.6 GHz and 4.2 GHz) whose frequency ratio is very large, for example, about 1 : 2.6 as in the case where it is utilized in the afore-noted wireless communication system.